The present invention relates to a polarizer suitable for improving the luminance of a liquid crystal display and preventing the uneven display thereof, and an optical element and a lighting device using the same.
Conventionally, as polarizers which can overcome the disadvantage of polarizing plates that about half of incident light is absorbed to result in absorption loss, thereby improving the luminance of liquid crystal displays, elements comprising laminated products of cholesteric liquid crystal layers and xc2xc wavelength plates have been known. In these elements, circularly polarized light transmitted through the cholesteric liquid crystal layers is converted to linearly polarized light by the xc2xc wavelength plates, and the linearly polarized light is allowed to be incident on the polarizing plates in agreement with polarization axes, thereby preventing absorption loss.
However, there is the problem that the luminance of the liquid crystal displays is not so much improved as expected, resulting in uneven display, probably because the linearly polarized light contains large amounts of light components absorbed by the polarizing plates. Further, when prism array layers are arranged for improving the luminance and the efficiency for light utilization by optical path control, there is also the problem that the luminance is largely reduced in a front direction (vertical direction).
It is therefore an object of the present invention to develop a polarizer, an optical element and a lighting device which can form polarized light having little absorption loss caused by a polarizing plate, can supply the polarized light to a liquid crystal cell with the excellent efficiency for incident light utilization, can form a liquid crystal display excellent in luminance and decreased in uneven display, and are little reduced in luminance in a front direction when a prism array layer is arranged.
According to the present invention, there is provided a polarizer having a function of separating incident light into polarized light through reflection and transmission, which has (a) a haze of 10% or less, or (b) a ratio of parallel transmitted light to incident light of 40 to 60%, (c) a ratio of mirror reflected light to the incident light of 25% or more, (d) a ratio of diffusion reflected light to the incident light of 20% or less or (e) a fraction of mirror reflected light to the whole reflected light of 60% or more.
Further, the present invention provides an optical element, in which a xc2xc wavelength plate formed of at least one retardation layer is provided on the above-mentioned polarizer, and a polarizing plate containing a dichroic substance is disposed thereon if necessary.
Still further, the present invention provides a lighting device comprising the above-mentioned polarizer or optical element disposed on a surface light source.
Furthermore, the present invention provides a liquid crystal display in which a liquid crystal cell is arranged on the light outgoing side of the lighting device, with a polarizing plate intervening therebetween.
In the present invention, the term xe2x80x9cparallel transmitted lightxe2x80x9d means transmitted light in the parallel state when parallel light is incident on a polarizer, and the term xe2x80x9cdiffusion transmitted lightxe2x80x9d means transmitted light not in the parallel state in the above-mentioned case. Further, the term xe2x80x9cmirror reflected lightxe2x80x9d means light satisfying the reflective law, taking reflection by the whole polarizer as reflection based on the ideal mirror surface, and the term xe2x80x9cdiffusion reflected lightxe2x80x9d means light not satisfying the reflective law. Furthermore, the term xe2x80x9cfraction of mirror reflected lightxe2x80x9d means the ratio of mirror reflected light to the whole reflected light, and is calculated by the following equation:
[mirror reflected light/(mirror reflected light+diffusion reflected light)]xc2x7100 
The ratio of parallel transmitted light, (parallel transmitted light/incident light)xc2x7100, is hereinafter also referred to as the parallel light transmittance; the ratio of diffusion reflected light, (diffusion reflected light/incident light)xc2x7100, as the diffusion transmittance; the ratio of transmitted light, (the total transmitted light/incident light)xc2x7100, as the total light transmittance; the ratio of mirror reflected light, (mirror reflected light/incident light)xc2x7100, as the mirror reflectance; the ratio of diffusion reflected light, (diffusion reflected light/incident light)xc2x7100, as the diffusion reflectance; and the fraction of mirror reflected light as the mirror reflection fraction. In the above, incident light=transmitted light+diffused light, and transmitted light=parallel transmitted light+diffusion transmitted light.
The polarizer of the present invention is excellent in mass production and separation function of polarized light, and provides the polarized light having little absorption loss caused by the polarizing plate with the excellent efficiency for incident light utilization. Further, it can form the optical element and the lighting device little reduced in luminance in a front direction, and the liquid crystal display excellent in luminance and decreased in uneven display, even when the prism array layer is arranged.
Further, minimization of the dispersion of the haze, the parallel light transmittance, the total light transmittance, the diffusion transmittance, the mirror reflectance and the diffusion reflectance in a plane can reduce uneven display such as uneven color, in addition to homogenization of luminance, thereby forming the liquid crystal display having good brightness and visibility.
The above-mentioned effects are based on the specified values of the haze, the parallel light transmittance, the mirror reflectance, the diffusion reflectance or the mirror reflection fraction, and the mirror reflectance, the diffusion reflectance and the mirror reflection fraction result from the following elucidation. That is to say, as a result of intensive investigation for solving the above-mentioned problems, the present inventors have elucidated that when a cholesteric liquid crystal layer having a constant thickness is combined with a xc2xc wavelength plate to form an optical element, the mirror reflectance, the diffusion reflectance and the mirror reflection fraction of the cholesteric liquid crystal layer have an influence on the luminance, and are largely related to the total reflectance, (reflected light/incident light)xc2x7100.
That is to say, the cholesteric liquid crystal layer shows mirror reflection characteristics containing no diffusion reflected light by surface reflection on both sides of the layer, when not oriented. However, the reflectance is low, and the total reflectance increases by orientation. It is therefore possible to obtain half of incident visible light as reflected light for natural light (the total reflectance: 50%). However, in the course of an increase in the total reflectance, diffusion reflected light is generated to form reflected light in which the diffusion reflected light and mirror reflected light are present as a mixture. Moreover, the mixing ratio thereof changes with an increase in the total reflectance.
With respect to the above-mentioned change, the diffusion reflectance increases and the mirror reflectance decreases, probably because the diffusion reflected light is predominantly generated at the beginning of an increase in the total reflectance. On the other hand, when the total reflectance further increases, the mirror reflected light is probably predominantly generated in its turn. Consequently, the mirror reflected light starts to increase while the diffusion reflectance is decreasing, reaching the mixing ratio giving the mirror reflectance higher than the diffusion reflectance.
The present inventors have elucidated that the polarizer having a mirror reflectance of 25% or more, a diffusion reflectance of 20% or less or a mirror reflection fraction of 60% or more is effective for an improvement in luminance in the above-mentioned change in the mixing ratio. Based thereon, it is conceivably advantageous to show reflection characteristics of the light containing a large amount of mirror reflected light and a small amount of diffusion reflected light, in terms of effective utilization of the light bringing about an improvement in luminance. Further, the present inventors have also elucidated that the in-plane scatter of the above-mentioned mirror reflectance, diffusion reflectance or mirror reflection fraction contributes to uneven luminance or uneven color. The degree of the uneven luminance or uneven color caused by the scatter is more increased in an oblique direction than in a front (vertical) direction. The uneven display or uneven color can be largely decreased by reducing the above-mentioned scatter to 10% or less for the mirror reflectance or the diffusion reflectance, and to 15% or less for the mirror reflection fraction.